1. Field of the Invention
The present invention relates to methods for determining airspeed. More particularly, this invention pertains to a method for measuring the horizontal airspeed of helicopters in low speed ranges.
2. Description of the Prior Art
Due to their design, helicopters exhibit increased instability at decreased airspeeds. This effect is particularly acute when hovering. This mode of low airspeed flight requires a very large amount of control action, increasing the responsibilities of the pilot accordingly. As the various control inputs depend directly on external disturbances (e.g. wind), the airspeed provides an important corrective. The precise determination and visual indication of airspeed is, however, quite difficult in the low speed ranges. Further, knowledge of the wind velocity is required for calculation of directional corrections necessary for the delivery of weapons from combat helicopters. Continual determination of the wind velocity additionally permits estimation of the velocity over the ground in the event of a Doppler radar failure.
In the high speed range (velocities exceeding 20 m/s), a conventional aerodynamic data system based on measured temperature and pressure differences (conventional in rigid wing aircraft) provides adequate airspeed measurement. However, such systems do not perform adequately at speeds below 20 m/s for a number of reasons. These include the limited working range (above 10 m/s) of commercial pitot-static tubes, perturbation effects caused by the rotor downwind and the multiple degrees of freedom of directional movement of helicopters (rearward, sideways and vertical flight).
Two technical solutions are currently utilized to determine airspeed (i.e. the velocity of the helicopter relative to the surrounding air) at speeds below 20 m/s. One of these is LASSIE (Low Air Speed Sensing and Indicating Equipment). This system is produced by the GEC Company (England) and is described by J. Kaletka in "Evaluation of the Helicopter Low Air Speed System LASSIE", Rotorcraft and Powered Lift Aircraft Forum, (Garmisch-Partenkirchen 1982). The other system is known as LORAS (LOw Range Air Speed System) and is produced by the PACER COMPANY of the United States. That system is described by P. J. Onksen in "Helicopter Omnidirectional Air Data Systems," IEEE 1983. Both of these systems provide mechanical solutions based upon conventional air data technology (differential pressure measurement and temperature measurement).
The LASSIE system is based upon classical air data measurement using the pitot-static tube and includes a probe that can be rotated about two axes. The probe is provided with a small tailplane for correct alignment in the air stream. In LASSIE, the yaw angle .beta. and the incidence angle .alpha. are measured and, in combination with temperature and pressure difference measurements, the velocity V is determined.
The essential feature of the measuring arrangement in the LORAS system, on the other hand, is a double-armed probe that rotates with constant velocity above the rotor. A solid connection between the probe and the helicopter casing is provided by a stationary tubular column supported in the rotor shaft. The two major arms have a diametral location. A tangentially sensitive venturi tube is located at the end of each arm. Each sensor is connected to a differential pressure gauge. If the probe rotates with constant velocity, no pressure difference is detected in hovering flight and a cyclic difference signal is detected with a phase position that corresponds to forward or sideward flight. The amplitude of the signal is a measure of horizontal airspeed, while the phase gives the incident flow direction.
The LASSIE and LORAS systems involve mechanical solutions in which the airspeed is measured externally. That is, the actual measuring elements must be mounted on the outer skin of the cabin or, after passage through the rotor shaft, above the rotor. Externally located measuring elements not only require special maintenance and care, but are particularly sensitive to damage. In military applications, efforts are commonly made to avoid externally located measuring devices because of their high vulnerability.